It is well known in the art that anhydrides of carboxylic and dicarboxylic acids may be produced by catalytic oxidation of aromatic hydrocarbons such as benzene, o-xylene, naphthalene, or of unsaturated aliphatic hydrocarbons such as butadiene, n-butene, and mixtures thereof. In general, phthalic anhydride production is achieved by reacting naphthalene or o-xylene vapors with an oxygen-bearing gas in a reactor, under appropriate temperature and pressure conditions and in the presence of a suitable catalyst, e.g., a vanadium pentoxide-bearing catalyst. The phthalic anhydride produced by the reaction is contained in the reactor effluent and is subsequently separated out and recovered.
FIG. 1 shows in more detail a conventional system for the production of phthalic anhydride by catalytic oxidation of naphthalene. Oxygen-bearing air is compressed by a compressor 2 driven by a motor 3 and is sent to an air receiver 4. A stream of air from air receiver 4 is heated to about 300.degree. F. in an air heater 6 and then enters the bottom of a fluidized catalytic reactor 8. This air passes through a grid 10 into a vanadium pentoxide-bearing catalyst bed 12 which is disposed on grid 10. The air entering the bottom end of the reactor is pressurized to about 37 psig.
Molten naphthalene is pumped from a storage tank 14 by a metering pump 16 through a vaporizer 18 where the naphthalene is vaporized before being injected into catalyst bed 12. Bed 12 is fluidized by the air and the primary oxygen/naphthalene reaction takes place in contact with catalyst bed 12. The rate at which the molten naphthalene is introduced into bed 12 is coordinated with the rate at which the oxygen-bearing air is introduced into bed 12 so that the reactor's catalyst bed 12 receives air and naphthalene in a ratio of about 10:1 by weight. Bed 12 is maintained at a reaction temperature of between about 600.degree. and about 750.degree. F. by means of suitable temperature control elements 20.
Reacted gases flow upward out of the top of fluidized catalyst bed 12 to a catalyst disengagement zone 22 where any catalyst particles which may have been carried upward from bed 12 by the rising gases disengage from the gases and settle back down onto bed 12. The reacted gases continue to rise and pass through a grid 24 and into a vanadium pentoxide-bearing catalyst bed 26 which resides on grid 24. Bed 26 is in turn fluidized by these gases. Catalyst bed 26 serves as a quench bed for the reacted gases, and to this end bed 26 is maintained at the appropriate quench temperature of about 525.degree. F. by means of temperature control elements 28. The latter, like temperature control elements 20, may be individually controlled U-shaped tubular heat exchangers carrying a suitable heat exchange fluid. The quenched gases thereafter pass from fluidized catalyst bed 26 to a disengagement zone 30 where catalyst particles which may have been carried from bed 26 by the rising gases will tend to disengage from the gases and settle back down onto bed 26.
A plurality of cyclones 32 are positioned at the top end of disengagement zone 30 and are intended to purge the reacted gases of any lingering catalyst particles before the gaseous effluent leaves the reactor and is processed to recover desired components. Each cyclone is provided with a dip-leg 34 for returning the separated catalyst fines back to catalyst beds 12 and 26.
The gas stream leaving cyclones 32 is passed through a gas cooler 36 where the stream is cooled to a temperature just above the dew point of phthalic anhydride, i.e. approximately 315.degree. F. The cooled stream then passes into a partial condenser 38 where up to approximately one-half of the phthalic anhydride that is present in the gaseous stream is condensed out as a liquid. The liquid phthalic anhydride is separated from the remaining gas stream and it flows via a line 39 to a liquid storage tank 40 for subsequent processing.
The gas stream exiting partial condenser 38 has a temperature of approximately 300.degree. F. It passes via a line 41 and a pressure control valve 42 to one of a battery of switch condensers 44a, 44b, 44c, etc. Switch condensers 44a, 44b, 44c, etc. are intended to remove the phthalic anhydride remaining in the effluent stream by sublimating the phthalic anhydride out of the reactor effluent as a solid. When a given switch condenser has condensed out a predetermined amount of phthalic anhydride, the switch condenser is shut off from the incoming gas stream and is switched over to a heating cycle to melt the condensed phthalic anhydride. Simultaneously, another switch condenser, which at this point has been re-cooled after completing its heating cycle, is opened to the gas stream to sublimate incoming phthalic anhydride. The phthalic anhydride which is melted in any of the switch condensers flows to a liquid storage tank 46 for further processing. The effluent stream that leaves a cooling switch condenser is sent to a gas scrubber 48 where it is washed with water before being vented to the atmosphere or to other process equipment for further treatment or recovery. The gas stream preferably leaves the switch condensers at a temperature of between about 125.degree. and about 140.degree. F. before being directed to the scrubber 48.
Pressure control valve 42, disposed in the gas line intermediate partial condenser 38 and switch condensers 44a, 44b, 44c, etc., imposes the back pressure on reactor 8 which is necessary in the practice of this invention. Also, by controlling the gas pressure at this point in the process the phthalic anhydride can be condensed in partial condenser 38 as a liquid instead of as a solid. This is so, because increased pressure can raise the dew point of phthalic anhydride above the melting point, thereby permitting condensation of the phthalic anhydride directly as a liquid. In the system described above, the pressure control valve is designed to hold the reactor top pressure at about 22 psig.
Unfortunately, a number of difficulties arise when a production process of the sort just described is utilized. In particular, it has been found that the cyclones 32 are incapable of removing all of the catalyst dust from the reaction gases before the gases leave the reactor. As a result, small amounts of catalyst dust (typically between about 0.007% and about 1.0% by weight) leave the reactor with the effluent and travel on to the downstream process elements. The dust in the gaseous effluent passes through gas cooler 36, partial condenser 38 and pressure control valve 42 and enters switch condensers 44a, 44b, 44c, etc. Inside the switch condensers, the catalyst dust in the reactor effluent creates complications when the condensers switch to their cooling cycle to sublimate out phthalic anhydride from the gas as a solid. In particular, the dust particles serve as a nucleus for condensing phthalic anhydride and allow some of the phthalic anhydride in the effluent to condense out of the effluent as a gas-born mist rather than sublimating substantially entirely as a solid deposited directly on the surfaces of the switch condensers. This is a problem since the gas-born mist tends to deposit and solidify on the switch condenser surfaces as a dense, slushy mass which is relatively unporous and which has a relatively poor heat transfer coefficient, whereas sublimated phthalic anhydride deposits on the switch condenser surfaces as a group of needle-like crystals which are relatively porous and which have a relatively good heat transfer coefficient.
The relatively low porosity of the dense, slushy phthalic anhydride deposition (vis-a-vis the relatively high porosity of the sublimation deposited solid phthalic anhydride) is of concern since it tends to increase the back pressure within the switch condensers. Such an increase in back pressure within the switch condensers is undesirable since it reduces the efficiency of the system and necessitates the use of a bigger and more expensive compressor 2 (and hence a bigger and more expensive motor 3) in order to make the system function where significant amounts of slushy phthalic anhydride deposition ocurs. In this regard it is to be appreciated that the principal power requirement of a phthalic anhydride production system of the sort shown in FIG. 1 is the power requirement of the motor 3. Thus, the capital costs and operational costs of a phthalic anhydride production system tend to increase where significant amounts of the relatively unporous slushy phthalic anhydride deposition occurs.
In addition, the relatively poor heat transfer coefficient of the slushy phthalic anhydride deposition (vis-a-vis the relatively good heat transfer coefficient of the sublimated porous solid phthalic anhydride) is of concern since it can (a) increase the amount of time necessary for cooling the effluent stream to the temperature required to solidify the phthalic anhydride, and (b) increase the amount of time necessary for melting out deposited phthalic anhydride from a full switch condenser. In addition, the relatively poor heat transfer coefficient of the slushy phthalic anhydride deposits can cause the heat exchangers to perform more work to recover the same amount of phthalic anhydride. As a result, production times and costs tend to be higher where significant amounts of slushy phthalic anhydride deposition occurs.
Since such dust-related complications threaten the economic viability of a plant using the aforementioned phthalic anhydride production system, many attempts have been made to eliminate the catalyst dust from the effluent prior to the effluent's receipt by the switch condensers.
One way of reducing the amount of catalyst dust carried into the switch condensers in the effluent stream is to replace the cyclones 32 with a more effective type of filtration system, i.e. screens, micro-metallic filters, or fiber filters. Such filtration devices can be more effective than cyclones in reducing the amount of catalyst dust in the reactor effluent, but they tend to create a more severe pressure drop in the system. Since such an increase in the system's back pressure tends to drive up the system's capital investment and power costs for the reasons specified above, such a solution is not entirely satisfactory.
Attempts have also been made to reduce the amount of catalyst dust reaching the switch condensers by inserting a liquid entrainment separator between liquid condenser 38 and the battery of switch condensers 44. Such a construction allows the removal of any dust-based mist at the entrainment separator. However, it has been found that such a solution is not entirely satisfactory since some catalyst dust still gets by the entrainment separator and is available to combine with liquid phthalic anhydride in the switch condensers so as to cause the undesired slushy phthalic anhydride deposition.